This invention relates to improvements in systems for restricting the telephone numbers which can be reached from a calling telephone.
U.S. Pat. No. 3,920,936 discloses a system for restricting the telephone exchanges which can be reached from a calling telephone unit. As is explained in said U.S. Pat. No. 3,920,936, there are situations in which it is desirable to restrict the telephone exchanges accessible in a pair of telephone lines. The primary reason for such restriction in commercial establishments with many phones and employees, is to limit telephone usage by employees to authorized exchanges and prevent them, or any unauthorized personnel, from dialing telephone exchanges with respect to which toll-call or long-distance charges are incurred. In private homes, it is often also desirable to limit the phone use to specified local exchanges and thereby prevent users in the home from making calls for which additional charges are incurred.
Briefly, in the embodiment disclosed in said U.S. Pat. No. 3,920,936, a read-only memory (ROM) is provided which stores three-digit combinations, each representative of a permissible telephone exchange or area code. The first three digits dialed by a telephone user are stored and digitally compared with the contents of the ROM. If a match is located, than the call is permitted to be completed in a conventional manner. If no match is found, a disabling signal is generated to disrupt the connection between the calling telephone unit and telephone lines, thereby preventing the user from reaching the dialed number.